The In Vitro Isolation of Murine Embryonic Stem Cells

Embryonic stem (ES) cells are derived directly from those progenitor cells of early mouse embryos that subsequently form all of the tissues of the fetus itself and that under appropriate culture conditions, can be maintained continuously in an undifferentiated state in vitro (1 –3 ; see Note 1 ). When introduced into a mouse blastocyst or aggregated with morulae, however, the ES cells are capable of responding to in vivo developmental signals and participate in normal embryogenesis leading to the formation of chimeric offspring (4 ). Furthermore, ES cells are also capable of colonizing the trophectodermal and primitive endodermal lineages of the extraembryonic membranes (5 ,6 ) and of supporting complete fetal development, with newborn pups resulting from the aggregation of stem cells with tetraploid cleavage-stage embryos being 100% ES cell-derived (7 ). The ES cells may also colonize the germ line in chimeras and form fully functional gametes (4 ). Hence, in combination with gene targeting, ES cells provide a powerful approach for introducing novel genetic change into the mouse genome (8 ).